Rear Brake Light Problems

I have a strange problem with the brake lights on my 1999 GMC Savana.
When I have the lights turned off the brake lights function but when the running lights are switched on and I brake , the lower brake lamps go off instead of getting brighter.
At first I thought it might have the incorrect bulbs installed which are 1157 for brake and turn lamps, I have also tried 2057 with the same results.
The third brake light up top works fine.
Anyone have this problem before?:banghead:
Thanks in advance
Stephen

Welcome to AF, look for bad grounds in that circuit, if a trailer hitch elec connector back there, check those connections also.

Hi, on my 2000 Chev Express I just had to remove the ground wires where they enter each socket. They clip in and there was a little bit of play which was creating a short circuit. I had to cut away some of the plastic on the socket where the ground wire enters. After cleaning up the oxidized metal on the socket and the ground wire clip, i decided to solder the connection together. Although a bit time consuming, I now have the use of all my brake lights, signal lights and parking lights. Make sure to electrical tape like crazy since it is on the outside of the truck where water can get into the socket. This is the reason for the problem in the first place.

I have the same issue on my van.Did it help?